The present invention relates to miniature inductors and pertains particularly to improved trimmable inductors and methods of manufacture.
Miniature inductors are widely used in UHF and microwave radio frequency electronic circuits. The inductors are made in two basic configurations: wire wound and monolithic. Monolithic inductors typically employ thin conductive film in several configurations, some of which are the flat spiral type formed on a planar substrate and the helical type formed on an elongated substrate.
A spiral-type inductor usually consists of a flat ceramic substrate on which single layer flat spiral metal pattern is defined. Other examples include stacked turns in a multi-layer construction; spirals situated on both sides of a thin substrate and spirals connected in series.
A helical-type monolithic inductor consists of an elongated ceramic or ferrite core on which is formed a conductive metal helical pattern. The substrate is covered with one or more metal layers which are then etched or cut to form a conductor in a helical fashion. Such a cut defines a helical conductive winding similar to a wire coil.
Often after an inductor is placed in a circuit, it does not perform precisely as expected due to influence of extraneous factors such as nearby components, construction of the printed circuit board, geometry of connections to the inductor, mounting technique, and the like. Also, sometimes the intended circuit operation is not achieved with the selected inductor value due to variation in values of other components in the circuit.
A common way to adjust component values in a circuit is to have adjustable components. While trim resistors and capacitors are common and available in reasonably small physical sizes, adjustable inductors tend to be quite large and therefore virtually incompatible with modern miniature electronic systems, such as portable cellular telephones and pagers, for example.
Adjustable components are more expensive than fixed-value components and their use often results in increased size, as well as costs, of the circuits. Adjustable components are also generally considered not as reliable as the fixed components.
The permanently in-circuit laser-trimmed components bridge the gap between adjustable and fixed-value components. However, heretofore the laser trimmable components were limited to resistors and capacitors only.
Some applications require the inductor to be of a certain precise value for the circuit to work. Customarily, for small surface mount inductors, if inductor of a given value does not perform satisfactorily, it is removed from the circuit by de-soldering and new inductors of different values are substituted instead, until satisfactory circuit performance is attained. Since commercial inductors generally come in standard values, there is a chance that the needed inductance will fall between the standard values available, resulting in sub-optimal performance of the circuit.
Additionally, in some applications, such as a Radio Frequency (RF) filter, it is advantageous to be able to change the Q-value of the circuit without major re-design or repeated layout and fabrication of the circuit.
Therefore, it is desirable that a miniature reliable and inexpensive trimmable inductor be available.